Filtering a substrate through a skein of fibers has long been known in the art. To date, the emphasis has been on maintaining the fibers clean by backwashing them (through their lumens) frequently, either with permeate, or clean water when the substrate is aqueous. Hereafter reference to "permeate" is because it is preferably used, but fresh, clean substrate or water may also be used, if more convenient. Alternatively, the fibers are pressured in their lumens, at frequent intervals, by gas under sufficient pressure to dislodge solids fouling the outer surfaces of fibers in the skein (see U.S. Pat. No. 4,655,927 to Ford). Only recently have skeins been operated for acceptably long periods while being aerated, and even in the case where a skein is arcuate, coarse bubbles were found to be more effective than fine (see U.S. Pat. No. 5,248,424, to Cote et al, col 9, lines 1-9, and col 11, lines 67-68); but the criticality of maintaining substantially equal flow of a stream of coarse bubbles through each of plural orifices in an aerator, irrespective of the orientation of the skein in the substrate, was not known.
Therefore it is not surprising that the art did not consider how to maintain a scrubbing aerator in a sufficiently clean condition to provide the necessary uniform flow. Specifically, the many references teaching microfiltration with a skein of fibers, failed to suggest how to clean clogged orifices in an internally fouled aerator in situ, so as to maintain the skein awash in a uniformly flowing stream of coarse bubbles.
An orifice through which air exits the lateral walls of an aerator, becomes clogged because solids penetrate the interior of the aerator and are deposited on its inner walls forming a sludge or slime which gradually accumulates on the periphery of the orifice. As air continuously passes through the orifice, the sludge is dried, providing a base for the accumulation of yet more sludge which, in turn, is dried, until the orifice is substantially plugged. As the diameter of the orifice is reduced, the size of the bubbles decreases in size, but they also rise more slowly than coarse bubbles, resulting in diminished effectiveness in scouring the essentially vertical, smooth surfaces of the fibers.
Using the method taught herein, a skein of fibers, most preferably in the vertical orientation, is kept continuously operational at equilibrium flux for long enough to be economical in commercial installations. Since fouling of the fibers is exacerbated when enough scrubbing air does not contact the fibers, it is essential that the orifices of the skein's aerator be kept open to provide adequate scrubbing. By flushing a fouled scrubbing aerator at frequent intervals with a specified mixture of permeate (or clean water) and air, optionally also with permeate and/or air, deposits loosened from within the aerator are directly returned to the substrate from which they were removed and the orifices are kept clean which in turn keeps the fibers clean, and operation economical.
Aerators which require cleaning during use are mainly used in the treatment of municipal sewage as described in U.S. Pat. No. 3,153,682 to Walker, inter alia. However, the purpose of such aeration is to provide mixing of the contents of a tank while aerating its contents, not to produce coarse bubbles for the specific purpose of scrubbing fibers. Further, such aeration produces a circulating roll of the contents in the tank (see col 1, lines 11-13). Air was supplied through large orifices in combination with a stream of water against an impingement surface so the air is broken up into fine bubbles and "clogging of the air orifices is virtually never encountered" (see col 1, lines 36-43). To maintain the desired clean orifices for satisfactory operation an orifice velocity of 126 ft/sec (38.4 meters/sec) with a 7" water column head loss, is specified (see col 3, lines 23-27), stating that it is desirable not to let the velocity fall below 70 ft/sec (21.34 m/sec). In his later U.S. Pat. No. 3,242,072 Walker emphasized the importance of maintaining a velocity of 70 ft/sec to prevent "creeping in" of the liquid sewage, and resultant growths' (see col 3, lines 62-64). To ensure adequate margin for maintaining the required air pressure Walker provided a pressure leg, down-flush leg, or blow-off (identified by numeral 31 in FIGS. 1, 2 and 4). To clean the aerator, Walker stops operation to fill risers 16 with liquid to the level of the liquid in the tank so that the entire air pressure is initially available to move the water and force it through a passage in blow-off 31, so that the water flows fast with a scouring effect (see col 4, lines 23-28).
Later, in U.S. Pat. No. 3,501,133 to Drier et al, they again shut off the air supply to flush sludge out of their spargers 24 (see col 4, lines 20-43). To avoid discharging sludge through the orifices 31 they provided "down-flush tubes" extending from the low point of each sparger. As before, the spargers were positioned to contribute to the same over-all rolling movement of the tank's contents being careful not to lose the characteristic of isolated air-lift columns which provided aeration efficiency.